1. Field of the Invention
The present invention relates to a method of casting iron around a highly hard steel member and a mold for carrying out such a method.
2. Description of the Relevant Art:
Trimming dies are known as devices for cutting off a workpiece such as an iron sheet or the like to a prescribed shape. The trimming dies generally include a lower die and an upper die movable toward and away from the lower die. The upper die is lowered toward the lower die, and while holding the workpiece against the lower die with a pad mounted on the upper die, the workpiece is severed to the desired shape by cutters on the upper and lower dies.
Since the cutters of the trimming dies are forcibly driven to sandwich and cut off the workpiece vertically, they are required to be made of highly hard steel material such as high carbon steel. The trimming dies, on the other hand, should preferably be made of cast iron which is inexpensive and tough in view of the cost of manufacture and machinability after the casting process.
There is known a method of manufacturing a composite product, such as trimming dies as described above, which is made of two metals having different physical properties such as hardness and toughness. According to this known method, after a trimming die has been manufactured by casting, a cutter is formed by padding or inserting on the die. However, the efficiency and cost of production are low because the cutter must be formed by a separate process after the die has been cast.
As a solution to the above shortcoming, a single process of manufacturing a composite product is known as disclosed in Japanese Laid-Open Patent Publication No. 57-25276.
According to the disclosed method, as shown in FIG. 11 of the accompanying drawings, a mold cavity 201 for molding a trimming die is defined in a mold 200 having separate runners 202, 203 communicating with the mold cavity 201. The mold cavity 201 is supplied with a molten mass of ordinary iron 205 through the runner 203 and with a molten mass of highly hard steel 204 for integrally casting a trimming die of materials having different material properties.
Where molten masses of materials of different natures are introduced into the mold cavity through the respective runners, a process known as two-layer casting, the different materials must be heat-treated in different manners after they have been cast. More specifically, the highly hard steel must be heat-treated at a higher temperature than the temperature at which the oridinary iron is heat-treated. However, it is difficult to heat-treat the different sections of the cast product under different temperature conditions. If the different sections were heat-treated differently, the cast-iron product portion would tend to have an undesired metal structure, resulting in material deterioration.
Another conventional process for making a composite product is known as a full mold process. In the full mold process, a pattern of a material such as foamed polystyrene which will evaporate upon heating and a member of highly hard steel to be cast in are set in a mold, and molten metal such as iron is poured into the mold to cause the pattern to evaporate, whereupon the molten iron is cast around the steel member.
If the heat mass of the steel member is large, then the steel member and the molten iron will not adhere well to each other. A method for eliminating this drawback is proposed in Japanese Patent Publication No. 49-7299.
FIG. 12 of the accompanying drawings shows such a proposed method. A member 301 of highly hard steel to be cast in is set in a mold cavity 302 defined in a mold 300, and a heating member 303 is disposed in the mold 300 so as to extend across the member 301 and the mold cavity 302. Molten iron poured through a sprue 304 into the mold cavity 302 is brought into contact with the heating member 303 which is then ignited and burned. The temperature of the member 301 is increased by the heat generated when the heating member 303 is burned, so that the member 301 and a main product portion formed by the molten iron will adhere well to each other. A gas that is given off when the heating member 303 is burned is discharged through a gas release hole 305.
With this method, however, the molten iron introduced into the mold cavity 302 contacts the member 301 before the temperature of the member 301 is increased, thus allowing the temperature of the molten iron at the confronting surface of the member 301 to drop sharply. As a result, a solidified film is formed at the interface between the member 301 and the product portion formed by the molten iron, resulting in a low mechanical strength of the interface. Moreover, inasmuch as the molten iron and the heating member are in direct contact with each other, they stick to each other due to burn-on, and the cast product cannot easily be removed from the mold.
When a loop-shaped member to be cast in is set in the mold and then molten iron is poured into the mold, the loop-shaped member will be expanded radially due to the heat of the molten iron and thereafter will shrink. Since the loop-shaped member which is made of highly hard steel is forced to shrink while being surrounded by the main product portion formed by the molten iron, the loop-shaped member tends to suffer from residual stresses which will be responsible for possible strains in the cast product.